IRREGULAR MIGRATION 97 



Ireland, the Adriatic Sea, northern Italy, across 

 France, and north into Sweden. By February, 1864, 

 the unusual visitors had disappeared. Though a few 

 pairs were known to breed, the majority in the 

 European invasion seem to have been merely 

 vagrant. 



In 1888 there occurred another irruption of the 

 Pallas's sand-grouse that was even more important. 

 Flocks spread throughout western Europe, and the 

 birds became so firmly established in the British 

 Islands that a special act of Parliament was put into 

 effect to protect them during an adequate closed 

 season. Subsequently, however, they disappeared. 

 Except to a few Russian naturalists who had ob- 

 served it in its Asiatic haunts, the species was prac- 

 tically unknown, until 1848, when it was found in 

 south Russia, and 1859, when it spread into western 

 Europe. It seems, therefore, to have been in a state 

 of unrest for the past seventy-five years or more, 

 but has not succeeded in establishing itself per- 

 manently in the new territory to which it spread. 

 Without interference by man it might possibly 

 have been more successful. It has been supposed 

 that the movements of sand-grouse, like those of 

 lemmings, are due to surplus production of individ- 

 uals, leading to undue congestion in the native 

 haunt. 



Other birds are subject to similar movements, as 



